But most successful pregnancies implant eight, nine or ten days after the egg's release from the ovary, with those implanting on day eight having the greatest chance of success.

"This is a very basic piece of reproductive biology that will probably find its way into textbooks," said the paper's lead author, Dr Allen Wilcox.

"This is the first really concrete information we have about when pregnancy starts in humans - how long after fertilisation it begins."

The researchers speculate that late-implanting embryos may be weaker and that rejection of them may be a protective mechanism designed to spare the mother the physiologic burden of supporting non-viable offspring.

While it has been known for more than ten years that 25 per cent of fertilised eggs fail to survive six weeks, the new studies, which monitored the hormones in daily urine samples of healthy women trying to conceive, showed the day by day trend for the first time in a large sample (221 women).

The work showed 13 per cent of pregnancies failed if implantation occurred by the ninth day, but the figure rose to 26 per cent when implantation occurred on the 10th day, 52 per cent on the 11th day and 82 per cent thereafter.

The researchers found no association between late implantation and clinical miscarriages that occurred later in pregnancy.

Australian reproductive expert, Dr Robert Jansen, believes there may be some technical problems with the study but that it nonetheless provides very valuable information, especially for IVF doctors.

"For me, the most startling fact was that 18 per cent of embryos implanted after the 11th day still produced healthy babies," said Dr Jansen, who is the head of reproductive medicine at Sydney's Royal Prince Alfred Hospital and director of the private company Sydney IVF.

"It just goes to show you shouldn't write off an embryo that's gone too slowly."